1. Field of the Invention
The present invention relates generally to thin-film electrical circuit structures and to methods for making such structures. More particularly, the invention is concerned with the provision of thin-film electrothermal switches and other electrothermally-actuated micromechanical devices.
2. Description of the Prior Art
Thin film deposition and patterning have been used for many years to fabricate hybrid electronic circuits and the passive electrical components (resistors, capacitors, inductors) included in such circuits. Certain active devices (e.g., field-effect transistors) also have been made using thin-film technology. However, circuit switching functions at present must be implemented using either conventional mechanical or electromechanical devices, or solid state switches. The former are normally much too large to be mounted on a thin-film circuit substrate, require long leads for connection to other circuit elements (a serious disadvantage in high frequency applications), and are relatively expensive in comparison to thin-film circuit components. The latter lack the characteristically high off-state to on-state impedance ratio of mechanical switches, and, for many applications, have undesirably high values of on-state "contact" resistance and off-state coupling capacitance.
In a variety of present-day and projected thin-film circuit applications, a need exists for low cost, microminiature switching devices that can be fabricated on conventional hybrid circuit substrates using thin-film deposition and patterning procedures compatible with those used to form the conductive paths, contact pads and passive circuit elements included in such circuits.
U.S. Pat. No. 3,763,454, issued Oct. 2, 1973 to Raymond A. Zandonatti and assigned to the assignee of the present invention, describes a thermally actuated safety switch for a hybrid solid state circuit. The switch includes a curved leaf spring that bridges a pair of conductive runs on the circuit substrate. The spring is welded to one run and joined, under tension, to the other run with a low melting point solder. If the current through an adjacent resistor exceeds a predetermined safe level, the heat it generates will melt the solder and release the spring, opening the circuit. Although the Zandonatti switch is useful for thick-film hybrid circuit applications, the nature of its construction makes it unsuitable for use in thin-film circuits or in other, general purpose applications.
A paper by K. E. Petersen, "Micromechanical Membrane Switches on Silicon", IBM J. Res. Develop., Vol. 23, No. 4, July 1979, describes the structure and operation of several micromechanical switching devices fabricated using conventional photolithographic and integrated circuit processing techniques. The disclosed structures are cantilever beams composed of thin (0.35-.mu.m) metal-coated insulating membranes attached to a silicon substrate at one end. The beams are suspended over shallow rectangular pits produced by anisotropic etching of the substrate. Actuation is achieved by applying a d.c. voltage between the metal coating on the membrane and a highly doped silicon layer at the bottom of the pit. This produces an electrostatic force on the cantilever beam, causing it to bend downward and bring a plated metal projection at its free end into contact with an underlying fixed electrode.
The micromechanical switches disclosed by Petersen are extremely small, but are not well suited for integration into thin-film hybrid circuits. In addition, relatively high voltage (60-70 V.) switching signals are required to actuate the devices, making them incompatible with low voltage, current-driven semiconductor circuitry.